Models of atrial and ventricular arrhythmias will be studied in anesthetized dogs, and in isolated tissue of rabbit and dog hearts. Mathematical models of atrial arrhthmias will also be developed to assist in relating effects of geometry, conduction velocity, and refractory period to self-sustained rapid rhythms, particularly atrial flutter. Models of actual and "pseudo" supernormal conduction in A-V nodal and intraventricular transmission will be tested in the dog heart in situ, and the effect of spontaneous pacemaker activity on intraventricular conduction will be studied. Digitalis-induced "after-potentials" (transient depolarizations) and "after contractions" in Purkinje fibers and ventricular muscle will be studied in vitro, and attempts will be made to elicit related behavior in the heart in situ. In the intact animals, ouabain will be administered to a point where transient depolarizations might be expected (but not, of course, manifest with extracellular recording). The mechanical counterpart should, however, be demonstrable on records of intraventricular pressure and/or mural tension as recorded with a strain-gauge arch.